136 ALEXANDER L. BOUNCE 



part of the glycolytic system is probably an essential component of cell 

 nuclei. Their evidence that glycolysis does not proceed all the way to lactic 

 acid, but instead stops at pyruvate, is however not convincing. Apparently 

 hexokinase is not present in cell nuclei, ^^^ so that fructose diphosphate must 

 be used as substrate. More work on this point is desirable. 



Glycolytic enzymes have thus been found in cell nuclei isolated by three 

 different procedures, and it is reasonable to suppose that these enzymes 

 may be true nuclear constituents. Since in most cells a certain amount of 

 metabolic screening of oxygen from the nucleus by the mitochondria can 

 be expected, it may be that the glycolytic system is of importance in gen- 

 erating energy for metabolic processes occurring within the nucleus. The 

 opinion of Lang et al}"^^ that the nucleus derives energy from the hydrolysis 

 of ATP by adenosinetriphosphatase seems unfounded, although it is pos- 

 sible that the nucleus might import ATP from the cytoplasm, to be used 

 subsequently in furnishing energy for synthetic reactions through phos- 

 phate transfer. 



Certain miscellaneous oxidative enzymes have also been investigated in 

 regard to their presence or absence in cell nuclei. Catalase probably occurs 

 in liver cell nuclei* -^^ •*" but may not always be present in appreciable con- 

 centration in nuclei.-^ 



Choline oxidase was absent from liver cell nuclei isolated by Lan'^^ at 

 pH 6.0 in very dilute citric acid. However, the whole hydrogen transport 

 system and cytochrome oxidase would have been needed to demonstrate 

 the enzyme by the technicjue which he used. Christie and Judah'^s found 

 that two components of choline oxidase of rat liver were predominantly 

 mitochondrial enzymes. Xanthine oxidase was shown to be absent from cell 

 nuclei isolated in strong sucrose solution by Lang et a/.,'^" but this is a 

 water-soluble enzyme and the work should be repeated if possible with 

 nuclei isolated by the Behrens technique. 



Malic dehydrogenase was found by Bounce^" in Uver cell nuclei isolated 

 at pH 6.0 in very dilute citric acid, but in relatively low concentration. 

 This enzyme apparently occurs in mitochondria as well as in soluble su- 

 pernatant fractions, and it remains in some doubt whether it is a true 

 nuclear enzyme or not. 



In summary, it can be said that cell nuclei are almost certainly lacking 

 in a number of important oxidizing enzymes which are primarily mito- 

 chondrial constituents (cytochrome oxidase, succinic dehydrogenase, uri- 

 case, and D-amino oxidase) and are probably lacking in others (choline 

 oxidase and xanthine oxidase). The complete Krebs cycle certainly cannot 

 function in cell nuclei, owing to the lack of succinic dehydrogenase and 

 probably of other enzymes of the cycle. However, the glycolytic enzymes 



1" G. S. Christie and J. D. Judah, Proc. Roy. Soc. (London) B141, 420 (1953). 



